How to get all of Pokémon Go Castform forms

Castform forms are based entirely on the current weather you are experiencing. Though it's not an exact science (much the Pokémon Go weather effects system itself) broadly here is how you can expect to find each one:

Castform form

Weather conditions

Normal Castform

Clear weather, Cloudy weather, Windy weather

Sunny Castform

Sunny weather (even possibly at night)

Rainy Castform

Rainy weather

Snowy Castform

Snowy weather, Foggy Weather

Note once a particular Castform has spawned, it won't change form if the weather later changes.